This invention relates in general to the synchronization of horizontal sweep in a video display with input synchronization pulses and in particular to the use of a multivibrator combination to achieve improved sweep synchronization.
Video information is displayed by a television receiver on a raster which is scanned horizontally at a first rate, and vertically at a second, generally slower rate. Video information is presented as amplitude-modulated synchronizing pulses by which the raster scanning of the television receiver is synchronized with the video input information. For proper picture framing, it is required that the frequency and phase of oscillation produced by the horizontal sweep system be synchronized with the frequency and phase of the horizontal synchronization signal transmitted from the broadcast station.
The most common approach to signal synchronization in television receivers utilizes a phase locked loop in which a phase detector has horizontal rate synchronization pulses applied to one input and ramp signals related to horizontal deflection applied to the other input. The phase detector responds to these inputs and produces control pulses which are filtered and used to control a horizontal-rate oscillator at a frequency which is an average of the incoming synchronizing signals. The horizontal oscillator, in turn, drives the horizontal deflection generator producing recurrent trace pulses which are integrated to produce the recurrent ramp, or sawtooth, signal input to the phase detector. Operating limitations in a phase locked loop system arise, however, when input synchronization signals vary over a large frequency range. The variation of input synchronization signal frequency becomes a critical operating parameter when a video display is used in a non-television type of application. For example, a video display used in a computer terminal or in a data display presentation system may be required to interface with a great variety of input synchronization signals. The typical television receiver not only lacks the flexibility to interface with this great variety of input signals, but also is incapable of controlling the presentation to fit a particular performance requirement, e.g., using the periphery of the display's raster to present data of one type and the center of the raster to present data of another type.
An improved approach to the synchronization of horizontal sweep in a video display with video synchronization input pulses is described and claimed in related patent application Ser. No. 077,517, filed Aug. 20, 1979, entitled "Horizontal Drive Circuit for Video Display," in the names of Richard Steinmetz and Gregory J. Beaumont, referred to earlier in the present application. The circuit disclosed therein utilizes a monostable multivibrator responsive to the leading, positive-going edge of the input synchronization pulses in combination with an astable multivibrator which is responsive to the trailing, or negative-going, edge of the monostable multivibrator output. The astable multivibrator locks on to the frequency of the input synchronization pulses and introduces a fixed delay between the start of synchronization and the initiation of the horizontal flyback pulse. In addition, the monostable multivibrator triggers the astable multivibrator to produce a variable delay between synchronization input pulse arrival and the initiation of the flyback pulse. the fixed delay output of the astable multivibrator is delivered to the horizontal drive transistor which delivers proper pulse timing to the horizontal output circuit. This unique multivibrator combination in a video display permits proper sweep synchronization over a greater range of synchronization signal pulse widths, e.g., 4-40 microseconds, and input frequency capture range, e.g., up to 1500 Hz. The present invention, by increasing synchronization signal injection to the astable multivibrator, permits an increase in the synchronization signal frequency capture range of up to 5000 Hz, or a more than three-fold increase in the previously attainable capture range.